Connected: An Internet Encyclopedia
RFC 1771 - 4.1 Message Header Format

Top: Connected: An Internet Encyclopedia
Up: Requests For Comments
Up: RFC 1771
Up: 4. Message Formats
Prev: 4. Message Formats
Next: 4.2 OPEN Message Format

### 4.1 Message Header Format

Each message has a fixed-size header. There may or may not be a data portion following the header, depending on the message type. The layout of these fields is shown below:

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                                                               |
          +                                                               +
          |                                                               |
          +                                                               +
          |                           Marker                              |
          +                                                               +
          |                                                               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |          Length               |      Type     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Marker:

             This 16-octet field contains a value that the receiver of the
             message can predict.  If the Type of the message is OPEN, or if
             the OPEN message carries no Authentication Information (as an
             Optional Parameter), then the Marker must be all ones.
             Otherwise, the value of the marker can be predicted by some a
             computation specified as part of the authentication mechanism
             (which is specified as part of the Authentication Information)
             used.  The Marker can be used to detect loss of synchronization
             between a pair of BGP peers, and to authenticate incoming BGP
             messages.

          Length:

             This 2-octet unsigned integer indicates the total length of the
             message, including the header, in octets.  Thus, e.g., it
             allows one to locate in the transport-level stream the (Marker
             field of the) next message.  The value of the Length field must
             always be at least 19 and no greater than 4096, and may be
             further constrained, depending on the message type.  No
             "padding" of extra data after the message is allowed, so the
             Length field must have the smallest value required given the
             rest of the message.

          Type:

             This 1-octet unsigned integer indicates the type code of the
             message.  The following type codes are defined:

                                        1 - OPEN
                                        2 - UPDATE
                                        3 - NOTIFICATION
                                        4 - KEEPALIVE

Next: 4.2 OPEN Message Format

Connected: An Internet Encyclopedia
RFC 1771 - 4.1 Message Header Format